The use of stack molds and manifolds in a hot runner injection molding apparatus is well known. Furthermore, it is well known that in many applications it is important that a runner layout be provided such that each cavity receives the same flow of melt having the same temperature and the same composition. Balancing of the runner system results in overall higher quality molded parts because consistency is achieved from mold cavity to mold cavity in a multi-cavity application. Even in multi-runner, single cavity applications, the benefits of balancing are well known and important.
A well-known technique for balancing a manifold or stack mold is to match runner diameters and lengths and to match the number of turns in the runners, so that the pressure drop through the manifold or stack mold to each cavity is the same. Occasionally, however, different flows are provided to different cavities, in spite of the runner layout having matched runner lengths and turns. This is at least partly due to a combination of shear heating of the melt flow combined with the layout of the runner system.
When melt is forced under pressure through a bore, as is done in a hot runner system whether in a manifold or a stack mold, the melt experiences friction or shear in the area adjacent to the channel wall. This results in a localized elevation of the temperature of the melt. The result is a differential in temperature across the bore, with the center of the channel being cooler than the material closer to the bore. Many hot runner systems split the melt flow from a primary runner through two or more secondary runners. When this occurs, the heat distribution profile in the melt is divided as well. This occurs because the flow through the runners is laminar, and therefore the shear-heated material remains adjacent to the wall as the corner is turned. After the corner, the heated peripheral portion is no longer annular, but is instead generally crescent-shaped and remains on one side of the melt flow. The mass flow through each of the secondary runners is substantially equal; however, the heated peripheral portion in each secondary runner is asymmetrically distributed about the periphery. If, as is usually the case, each secondary runner is divided into a plurality of tertiary runners, the asymmetric heated peripheral portion may be unequally divided between these plurality of tertiary runners. As a result, the material flowing into one of the tertiary runners from a secondary runner may include a higher proportion of shear-heated material compared to the melt flowing into the other of the tertiary runners downstream from that secondary runner. This phenomenon can, in some applications, cause preferential flow to some drop locations, and can cause out-of-spec product from portions of a molding machine. Specifically, there will typically be preferential flow to the tertiary runner receiving a higher proportion of shear-heated material from its upstream secondary runner compared to the other of the tertiary runners fed by that secondary runner.
This problem of asymmetric division of shear-heated material has been recognized in a cold runner context; however, it has not been as clearly recognized in a hot runner context. That is, melt in a hot runner is typically less viscous than in a cold runner. As a result, shear-induced heating has been thought of as less of a problem, as there is less resistance to shear. Instead, imbalance of flow in hot runner context has been attributed to other factors.
Different devices have been developed to address the problem for both cold runner and hot runner applications. In cold runner injection, the mold component includes the runners as well as the mold cavities. The mold component is made up of two halves that mate together. All the runners and the mold cavities lie in the plane of the mating faces of the two halves. At the end of an injection cycle the mold component is parted and the molded parts and the solidified melt in the runners are ejected. Cold runner layouts are typically simple in nature since all the runners lie on a common plane.
For cold runner applications, U.S. Pat. No. 4,123,496 to Gallizia et al. discloses an equalization device in a conduit carrying a melt flow, wherein different portions of the melt flow are reoriented to achieve a relatively uniform heat distribution in the melt flow.
U.S. Pat. No. 6,077,470 to Beaumont discloses a similar device for achieving similar balancing results in cold runner applications. Beaumont discloses a device for achieving balanced melt flow in cold runner applications. The device is positioned upstream of the split that first produces an asymmetric flow. Beaumont's device applies to cold runners particularly because the device is dependent on the simple, planar nature of the cold runner mold. Beaumont's device, for example, would not be applicable in a situation in which one of the runners from a downstream split extended out of the plane of the parting line of the mold component.
Hot runner stack mold systems typically include a plurality of mold components, which taken together, define a hot runner system and a plurality of mold cavities. Similarly, hot runner manifolds provide a hot runner system for providing melt to a plurality of mold cavities. In a stack mold, the primary runner or upstream runner of the runner system is typically provided by passages in a first, second and third mold component. In such stack molds, the second mold component is between and adjoins the first mold component and the third component. In the third mold component, the primary runner divides into two secondary runners. One of these secondary runners proceeds back into the second mold component where it divides into a pair of tertiary runners. The other of the secondary runners projects into the fourth mold component where it divides into two tertiary runners. The tertiary runners in the second and fourth mold components then provide melt to cavities in the third mold component. In operation, the second, third and fourth mold components are separable to eject the formed product from the mold cavities. Because of the differences in hot runner and cold runner systems, cold runner technologies are not typically applied to hot runner molding machines. For hot runner systems, other devices have been developed.
European Patent Application No. 963,829 to Goldwin et al. discloses the use of cylindrical heaters positioned at different points in a hot runner manifold. The heaters are positioned around the runners themselves and heat the melt passing through the runners so that cooler portions of the melt flow are heated to a temperature similar to the shear-heated portions of the melt flow.
U.S. Pat. No. 5,683,731 to Deardurff et al. discloses a device for use in hot runner manifolds having a double X layout. The device separates the hotter portion of an asymmetric shear-heated melt flow and redistributes it into each runner of the X, so that the runners receive melt having roughly equal temperatures.
Also for hot runner systems, it is known to pass a melt flow through one or more static mixers positioned in the runners. This creates a relatively uniform heat distribution so that any downstream split in a runner system divides the heat content in the melt flow generally equally in the runners after the split. Many injection molders, however, perform color changes during production runs and cannot tolerate cross-contamination between successive colors. Static mixers have complex internal structures, and are therefore difficult and time consuming to clean, making them poorly suited to many injection molding applications, such as those where color changes are common and cross-contamination is not tolerated.
Accordingly, there is a need for a hot runner system that offers improved balancing of the resin melt flows while facilitating efficient resin color changes between molding operations.